1. Field of the Invention
This invention relates to a master information carrier carrying thereon an irregularity pattern representing the information to be transferred to a magnetic medium and a magnetic transfer method.
2. Description of the Related Art
With an increase in information quantity, there is a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc, a zip (Iomega) and the like. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At the present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer). However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication Nos. 63(1988)-183623, 10(1998)-40544 and 10(1998)-269566, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
However, the master information carrier employed in the magnetic transfer disclosed in above identified Japanese Unexamined Patent Publications comprises a base sheet having on the surface thereof an irregularity pattern representing the information to be transferred and a magnetic layer formed at least on the surface of the protruding portions of the base sheet and since the magnetic transfer is carried out with the master information carrier and the slave medium held in close contact with each other, the magnetic layer can be partly lost or peeled off the pattern while the master information carrier is repeatedly brought into contact with and moved away from a number of slave media.
Removal of the magnetic layer from the master information carrier deteriorates contact between the master information carrier and the slave medium, which can result in deterioration in quality of the transferred signal. Further when the magnetic layer is peeled off the master information carrier over a wide area thereof, it is necessary to change the maser information carrier. The master information carrier is expensive and how many slave media can be made per one master information carrier largely governs the production cost of a magnetic recording medium.
We observed master information carriers from which the magnetic layer had been peeled and found that the magnetic layer was most peeled off the master information carrier at edges of flat top surfaces of the protruding portions of the irregularity pattern on the master information carrier and that as the pressure under which the master information carrier and the slave medium were in contact with each other increased, the magnetic layer was peeled off the master information carrier at a higher frequency. That is, it was presumed that dynamic structural weakness of the master information carrier caused the magnetic layer to be peeled off. The dynamic structural strength of the master information carrier is governed by, for instance, the thickness of the magnetic layer and the bonding strength between the base sheet and the magnetic layer. Our further investigation has revealed that edges of flat top surfaces of the protruding portions of the irregularity pattern on the master information carrier were often cut where the side faces of the protruding portions were covered with a very thin magnetic layer or with no magnetic layer, that is, the thickness distribution of the magnetic layer generated a structural strength distribution, and stress was concentrated on the portions where structural strength was weak to generate peeling of the magnetic layer.
Further, our investigation has revealed that the thickness of the magnetic layer on the side faces of the protruding portions of the irregularity pattern on the master information carrier largely affects the quality of the signal transferred to the slave media.
In view of the foregoing observations and description, the primary object of the present invention is to provide an improved master information carrier for magnetic transfer which has an improved durability and can transfer information to slave media so that an excellent reproduction signal can be obtained in the slave media.
Another object of the present invention is to provide a magnetic transfer method using the improved master information carrier.
In accordance with a first aspect of the present invention, there is provided a master information carrier for magnetic transfer to an in-plane magnetic recording medium comprising a base sheet having an irregularity pattern representing information to be transferred to an in-plane magnetic recording medium and a magnetic layer formed along the irregularity pattern, wherein the improvement comprises that
the thickness da1 of the magnetic layer on the upper surface of a protruding portion of the irregularity pattern and the thickness da2 of the magnetic layer on each of the side surfaces of the protruding portion satisfy the following condition,
0.05 less than da2/da1xe2x89xa61.3.
In accordance with a second aspect of the present invention, there is provided a master information carrier for magnetic transfer to a perpendicular magnetic recording medium comprising a base sheet having an irregularity pattern representing information to be transferred to a perpendicular magnetic recording medium and a magnetic layer formed along the irregularity pattern, wherein the improvement comprises that
the thickness db1 of the magnetic layer on the upper surface of a protruding portion of the irregularity pattern and the thickness db2 of the magnetic layer on each of the side surfaces of the protruding portion satisfy the following condition,
0.1 less than db2/db1xe2x89xa60.5.
In the present invention, the information to be transferred to an in-plane magnetic recording medium or a perpendicular magnetic recording medium generally includes servo information, and it is preferred that the magnetic layer be a soft magnetic layer or a semi-hard magnetic layer weak in coercive force.
According to our investigation, in the master information carrier for magnetically transferring information to in-plane magnetic recording media, peeling of the magnetic layer often takes place when da2/da1 is not larger than 0.05 and durability of the master information carrier deteriorates. As the thickness da2 of the magnetic layer on each of the side surfaces of the protruding portion increases, peeling of the magnetic layer becomes less and durability of the master information carrier remarkably increases. However, when da2/da1 is larger than 1.3, the magnetic flux which should enter the slave medium from the magnetic layer on the upper surface of the protruding portion upon application of the transfer magnetic field becomes apt to be drawn in the magnetic layer on the side surface of the protruding portion and the intensity of the transfer magnetic field deteriorates, which results in deterioration of quality of signals reproduced from the slave medium after the magnetic transfer.
Accordingly, in the case of the master information carrier for magnetically transferring information to in-plane magnetic recording media, when 0.05 less than da2/da1xe2x89xa61.3, durability of the master information carrier is improved and an optimal magnetization pattern can be formed on the slave media so that high-quality reproduction signals can be obtained. When durability of the master information carrier is improved, a larger number of preformatted magnetic recording media can be obtained from one master information carrier, whereby preformatted magnetic recording media can be less expensive.
In the case of the master information carrier for magnetically transferring information to perpendicular magnetic recording media, durability of the master information carrier can be improved when db2/db1 greater than 0.1 as in the master information carrier for the in-plane magnetic recording media. However, in the perpendicular recording, the intensity of the magnetic field at magnetization inversion portions on the slave medium opposed to the protruding portions of the irregularity pattern on the master information carrier can be enhanced by concentrating magnetic fluxes on the magnetic layer on the side surface of the protruding portions, and our investigation has revealed that when db2/db1 less than 0.1, the transfer magnetic field intensity cannot be sufficient to obtain high-quality reproduction signals. However when db2/db1 greater than 0.5, the magnetic flux densities at the protruding portions become too small to obtain high-quality reproduction signals.
Accordingly, in the case of the master information carrier for magnetically transferring information to perpendicular magnetic recording media, when 0.1 less than db2/db1xe2x89xa60.5, durability of the master information carrier is improved and an optimal magnetization pattern can be formed on the slave media so that high-quality reproduction signals can be obtained. When durability of the master information carrier is improved, a larger number of preformatted magnetic recording media can be obtained from one master information carrier, whereby preformatted magnetic recording media can be less expensive.
In accordance with a third aspect of the present invention, there is provided a magnetic transfer method for magnetically transferring information carried by a master information carrier, comprising a base sheet having an irregularity pattern representing information to be transferred to an in-plane magnetic recording medium and a magnetic layer formed along the irregularity pattern, to an in-plane magnetic recording medium having a magnetic layer at least on one side thereof, the method comprising the steps of applying an initial DC magnetic field to the in-plane magnetic recording medium in a direction parallel to the tracks thereof so that the magnetic layer of the in-plane magnetic recording medium is magnetized in the direction and then applying a transfer magnetic field in a direction opposite to the direction of the initial DC magnetic field to the magnetic layer of the magnetic recording medium and the magnetic layer of the master information carrier with the magnetic layers held in a close contact with each other, wherein the improvement comprises that
the thickness da1 of the magnetic layer on the upper surface of a protruding portion of the irregularity pattern on the master information carrier and the thickness da2 of the magnetic layer on each of the side surfaces of the protruding portion satisfy the following condition,
0.05 less than da2/da1xe2x89xa61.3.
In accordance with a fourth aspect of the present invention, there is provided a magnetic transfer method for magnetically transferring information carried by a master information carrier, comprising a base sheet having an irregularity pattern representing information to be transferred to a perpendicular magnetic recording medium and a magnetic layer formed along the irregularity pattern, to a perpendicular magnetic recording medium having a magnetic layer at least on one side thereof, the method comprising the steps of applying an initial DC magnetic field to the perpendicular magnetic recording medium in a direction perpendicular to the track plane thereof so that the magnetic layer of the perpendicular magnetic recording medium is magnetized in the direction and then applying a transfer magnetic field in a direction opposite to the direction of the initial DC magnetic field to the magnetic layer of the magnetic recording medium and the magnetic layer of the master information carrier with the magnetic layers held in a close contact with each other, wherein the improvement comprises that
the thickness db1 of the magnetic layer on the upper surface of a protruding portion of the irregularity pattern on the master information carrier and the thickness db2 of the magnetic layer on each of the side surfaces of the protruding portion satisfy the following condition,
0.1 less than db2/db1xe2x89xa60.5. 
Preferably the ratio of the thickness of the magnetic layer on each of the side surfaces of the protruding portion to the length S of the recess (the minimum of the lengths of the recesses between the tops of adjacent protruding portions of the irregularity pattern as measured in the direction of track) is larger than 0.05 and smaller than 0.4, that is, 0.05 less than the thickness of the magnetic layer on each of the side surfaces of the protruding portion/the length S of the recess less than 0.4.
When the ratio is not larger than 0.05, the amount of magnetic flux converged on the magnetic layer becomes insufficient to ensure sufficient signal quality, whereas when the ratio is not smaller than 0.4, the intensity of the transfer magnetic field generated between protruding portions becomes insufficient to ensure sufficient signal quality.
It is further preferred that the angle xcex8 of inclination of each of the side surfaces of the protruding portion of the irregularity pattern (the angle between the protrusion side extension of the surface of the recess and the side surface) be not smaller than 30xc2x0 and smaller than 80xc2x0. When the angle xcex8 of inclination of the side surface is smaller than 30xc2x0, the transfer magnetic field generated from pattern edges is too dispersed to ensure sufficient signal quality, though the durability of the magnetic layer is greatly improved. Whereas, when the angle xcex8 of inclination of the side surface is not smaller than 80xc2x0, pressure history in magnetic transfer is concentrated between the flat portion and the magnetic layer on the side surface of the protruding portion, which results in generation of crack and peeling of the magnetic layer, though no problem is involved in quality of the signal.
The magnetic layer of a slave medium may be formed of Co, Co alloy (e.g., CoPtCr, CoCr, CoPtCrTa, CoPtCrNbTa, CoCrB, CoNI, Co/Pd), Fe or Fe alloy (e.g., FeCo, FePt, FeCoNi).
It is preferred to clearly transfer the information that the magnetic layer be large in magnetic flux density and has magnetic anisotropy in the same direction as the slave medium (the direction of in-plane in the case where the slave medium is an in-plane magnetic recording medium whereas a perpendicular direction in the case where the slave medium is a perpendicular magnetic recording medium).
It is preferred that the magnetic layer be provided with a non-magnetic primer layer, for instance, of Cr, CrTi, CoCr, CrTa, CrMo, NiAl, Ru or Pd which gives a necessary magnetic anisotropy to the magnetic layer.
In the case where the slave medium is a perpendicular magnetic recording medium, a backing layer of a soft magnetic layer, for instance, of NiFe, CoCr, FeTaC or FeAlSi may be provided under the non-magnetic primer layer in order to stabilize the perpendicular magnetization of the magnetic layer and to improve the sensitivity upon recording and reproduction.
The thickness of the magnetic layer is preferably not smaller than 10 nm and not larger than 500 nm, and more preferably not smaller than 20 nm and not larger than 200 nm.
The thickness of the non-magnetic primer layer is preferably not smaller than 10 nm and not larger than 150 nm, and more preferably not smaller than 20 nm and not larger than 80 nm.
The thickness of the backing layer is preferably not smaller than 50 nm and not larger than 2000 nm, and more preferably not smaller than 80 nm and not larger than 400 nm.